https://codereview.chromium.org/1762513002/

Use gclient's git_cache setting in bot_update/api.py

BUG=589140
R=szager@chromium.org,iannucci@chromium.org

Review URL: https://codereview.chromium.org/1784043002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@299260 0039d316-1c4b-4281-b951-d872f2087c98